cmd: mmc: Support mmc hwpartition user enh start -

Add option to extend the hardware partition to the maximum size by
using the '-' dash sign instead of $cnt parameter. This is useful
in case we want to switch the entire eMMC user area into pSLC mode,
especially in case the device may be populated with different size
eMMCs. With this change, we do not have to calculate the number of
blocks of the user area manually.

To switch the pSLC mode for user area, use e.g. the following.
WARNING: This is a one-time irreversible change.
=> mmc hwpartition user enh 0 - wrrel on complete

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>